Abstract:

<p>Barium (Ba) is a classic permeant blocker of potassium (K) channels. The &quot;external lock-in effect&quot; in barium block experiments, whereby the binding of external K impedes the forward translocation of the blocker, provides a powerful avenue to investigate the selectivity of the binding sites along the pore of potassium channels. Barium block experiments show that the external lock-in site is highly selective for K over Na. Wild-type KcsA was crystallized in low K conditions, and the crystals were soaked in solutions containing various concentrations of barium. Structural analysis reveals open and closed gate conformations of the KcsA channel. Anomalous diffraction experiments show that Ba primarily binds to the innermost site S4 of the selectivity filter of the open-gate conformation and also the site S2, but no binding is detected with the closed-gate conformation. Alchemical free-energy perturbation calculations indicate that the presence of a Ba ion in the selectivity filter boosts the specificity of K binding relative to Na in the external sites S0-S2.</p>